Published on Web 09/19/2007
Low-Temperature Rapid-Scan Detection of Reactive
Intermediates in Epoxidation Reactions Catalyzed by a New
Enzyme Mimic of Cytochrome P450
Natalya Hessenauer-Ilicheva,† Alicja Franke,† Dominik Meyer,‡
Wolf-D. Woggon,*,‡ and Rudi van Eldik*,†
Contribution from the Institute for Inorganic Chemistry, UniVersity of Erlangen-Nu¨rnberg,
Egerlandstrasse 1, 91058 Erlangen, Germany, and Department of Chemistry, UniVersity of
Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
Received May 8, 2007; E-mail: vaneldik@chemie.uni-erlangen.de
Abstract: The use of synthetic iron(III) porphyrins as models for heme-type catalysts in biomimetic
cytochrome P450 research has provided valuable information on the nature and reactivity of intermediates
produced in the “peroxide shunt” pathway. This article reports spectroscopic detection of reactive
intermediates formed in the epoxidation reaction of cis-stilbene with m-chloroperoxybenzoic acid catalyzed
by a new mimic of cytochrome P450 with a substituted RSO3- group (1). The application of low-temperature
rapid-scan stopped-flow techniques enabled the determination of equilibrium and rate constants for the
formation and decay of all intermediates in the catalytic cycle of 1, including the rate constant for the formation
(1•+)FeIVdO and for oxygen transfer to the substrate. Noteworthy, the reaction of (1•+)FeIVdO with cis-
stilbene leads to an almost complete re-formation (95%) of the starting complex 1. The results show that
complex 1 is a valuable catalyst with promising properties for further applications in a biomimetic approach
toward mimicking oxygenation reactions of cytochrome P450.
cation radical ((Por•+)FeIVdO, referred to as compound I )
Introduction
CpdI in the catalytic cycle of heme-containing enzymes) is
Elucidation of the mechanism of reactive intermediate forma-
tion in oxygenation reactions catalyzed by cytochrome P450
enzymes is essential to understand the chemistry of in vivo
processes and is of continued interest in biological and bioi-
norganic chemistry. Therefore, over the past three decades
biomimetic approaches toward mimicking the oxygenation
reactions of these enzymes have focused on synthetic iron(III)
porphyrin complexes and their interaction with oxygen donors
such as iodosylbenzene, peroxy acids, and hydroperoxides.1 It
was clearly demonstrated that the nature and reactivity of the
intermediates produced via a “peroxide shunt” pathway can
easily be tuned by (1) the electronic and steric properties of the
porphyrin moiety, (2) variation of the central metal atom or
axial ligands, (3) changing the reaction conditions (i.e., pH,
protic vs aprotic solvent, temperature), and (4) the chemical
nature of the oxidant used. Although it has been generally
implicated in the literature that an oxo-iron(IV) porphyrin
formed as a reactive intermediate in the reactions of iron(III)
porphyrin complexes with oxidants,1,2 direct characterization of
(Por•+)FeIVdO species has been very difficult because of their
high reactivity in subsequent reactions.
Recently, Woggon et al. synthesized two new enzyme mimics
for cytochrome P450 in which the RS- ligand is replaced by
an RSO3- group (complexes 1 and 2 in Figure 1).3 Substitution
of the S- donor in P450 by an RSO3- group in these complexes
significantly reduces the negative charge density on the fifth
axial ligand and remarkably tunes the redox potential of FeIII/II
3b
to that measured for the resting state of P450cam
.
Moreover,
although coordination by the thiolate ligand was changed to
the RSO3- group, complexes 1 and 2 were shown to be valuable
P450 models with respect to electrochemistry and displayed a
good reactivity toward alkene epoxidation (with a turnover
(2) (a) Nam, W.; Park, S.-E.; Lim, I. K.; Lim, M. H.; Hong, J.; Kim, J. J. Am.
Chem. Soc. 2003, 125, 14674. (b) Goh, Y. M.; Nam, W. Inorg. Chem.
1999, 38, 914. (c) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo,
T. E.; Evans, B. J. J. Am. Chem. Soc. 1981, 103, 2884. (d) Groves, J. T.;
Watanabe, Y. J. Am. Chem. Soc. 1988, 110, 8443. (e) Gross, Z.; Nimri, S.
Inorg. Chem. 1994, 33, 1731.
(3) (a) Woggon, W.-D.; Leifels, T.; Sbaragli, L. Cytochromes P450: Bio-
chemistry, Biophysics and Drug Metabolism, 13th International Conference
on Cytochromes P450, Prague, Czech Republic, June 29-July 3, 2003.
(b) Meyer, D.; Woggon, W.-D. Chimia 2005, 59, 85. (c) Woggon, W.-D.
Acc. Chem. Res. 2005, 38, 127. (d) Kozuch, S.; Leifels, T.; Meyer, D.;
Sbaragli, L.; Shaik, S.; Woggon, W.-D. Synlett 2005, 4, 675. (e) Sbaragli,
L.; Woggon, W.-D. Synthesis 2005, 9, 1538. (f) Meyer, D.; Leifels, T.;
Sbaragli, L.; Woggon, W.-D. Biochem. Biophys. Res. Commun. 2005, 338,
372.
† University of Erlangen-Nu¨rnberg.
‡ University of Basel.
(1) (a) McLain, J. L.; Lee, J. J.; Groves, T. In Biomimetic Oxidations Catalyzed
by Transition Metal Complexes; Meunier, B., Ed.; Imperial College Press:
London, 2000; pp 91-169. (b) Meunier, B. In Metalloporphyrins Catalyzed
Oxidations; Montanari, F., Casella, L., Eds.; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 1994; pp 1-47. (c) Shimada, H.; Sligar,
S. G.; Yeom, H.; Ishimura, Y. In Oxygenases and Model Systems; Funabiki,
T., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1997;
pp 195-221. (d) Watanabe, Y. In Oxygenases and Model Systems;
Funabiki, T., Ed.; Kluwer Academic Publishers: Dordrecht, The Nether-
lands, 1997; pp 223-282. (e) Traylor, T. G.; Traylor, P. S. In ActiVe Oxygen
in Biochemistry; Valentine, J. S., Foote, C. S., Greenberg, A., Liebman, J.
F., Eds.; Chapman & Hall: London, 1995; pp 84-187.
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10.1021/ja073266f CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 12473-12479
12473